A need exists in many industries for a stainless steel which takes a high polish while having substantial mechanical properties including hardness, wear resistance and machinability. An exemplary application is so-called lens quality steel. The term lens quality means, as used in this application and with reference to the plastic forming industry, a steel which, when formed into a mold, is capable of taking, and retaining over a long period of use, a high polish. Examples of products produced by molds and dies made from such steel are clear plastic sheets which are used to form dashboard covers, or lens, in automobiles, the face shields which form the front portion of fighter pilot head masks, glass television picture tubes and medical industry equipment such as specimen holders and viewing plates. The criticality of these applications can be appreciated from the known requirements of their stated use; dashboard lens must provide true, undistorted instrument readings; fighter pilot head masks must not have any distorting defect at any location within the lines of sight of the user; televisions must provide undistorted viewing, and medical lens and specimen holders must not have flaws which an observer could confuse with the subject under study, such as cell growth, contour, absence or presence, i.e., an observer should not confuse a lens distortion for the presence of cancer cells. For purposes of ease of description and understanding, the term lens quality will be used in conjunction with steel for molds and dies used to produce automotive dashboard lenses.
At the current time several producers have attempted to supply Type 420 (UNS-S4200) stainless steel for molds to produce lens quality products, which steel has a specified composition of:
______________________________________ C Mn P S Si CR ______________________________________ Over 1.00 0.040 0.030 1.00 12.00 .15 max max max max 14.00 ______________________________________
As noted in Structures and Properties of Engineering Alloys, William F. Smith, McGraw-Hill Inc., 1981, p. 285, the above steel and related steels such as 403, 410, 414, 422, 431, 440A, 440B and 440C is martinsitic. However, the chemical composition of such steels is limited since a minimum of 12 percent Cr is required for corrosion resistance. At this chromium level the maximum amount of carbon that can be added is about 0.15 percent or the excess carbon will precipitate carbides near the grain boundaries and lower the chromium content below the critical 12 percent level, thereby increasing the risk of corrosion. Further, the amount of alloying elements that can be added to martinsitic steel is limited since these elements, like carbon, tend to depress the M.sub.s transformation and, if the M.sub.s is depressed too low, austinite will be retained at room temperature. It has also been noted that the corrosion resistance of the martinsitic stainless steels is relatively poor compared to the austinitic and ferritic stainless steels. Most martinsitic stainless steels contain just the minimum 12 percent Cr required for passivity in moist air since, if more chrome was added, the formation of ferrite would be promoted at the expense of austinite, the latter being necessary for the formation of martinsite. The chemical composition of martinsitic stainless steels is designed for strength and hardness as well as corrosion resistance, and therefore the chemical balance for corrosion resistance in these alloys is poor.
As a consequence attempts have been made to use 420 stainless made by special processes, including electroslag remelting, and even double ESR techniques. The product produced by such a process is of course very expensive, and sales have been made at this writing at about $6 per pound. It has been discovered however that even this type of material has a high rate of rejection when used for molds which produce lens quality products such as TV screens and automotive dashboards. Such molds are polished to a very high degree and are required to take and hold such a polish for long production runs in a pressure die process.
Defects which can cause rejection are often attributable to the formation of chromium carbides in the steel which, in effect, underlies the mechanism of corrosion. Thus, it is desirable to keep Cr in solution at all times and in all phases of the steel. Should chromium carbides precipitate in the local areas at grain boundaries, the surrounding matrix is depleted of Cr and therefore, since the steel preferably is at or near its lowest Cr content to attain stainless characteristics, corrosion will occur in the Cr depleted areas or regions. This concept is often referred to as sensitization and for the conventional 12 percent Cr steel, corrosion inevitably follows if the carbon is about 0.3 percent since this amount of C will result in the formation of chromium carbides during the steel making process. It should also be noted that sensitization often occurs in welding, and can even occur during processing stages including solidification and/or heat treatment. It should also be noted in this connection that sensitization is used, not in the sense of denoting stainless characteristics, but rather in the sense of maintaining stainless characteristics more readily with a given Cr content.
Early attempts to reduce sensitization included forming steel with substantial contents of V and Nb and moderate amounts of Mo, or of large quantities Mo, but the resultant product did not produce a satisfactory steel. In the case of steel with substantial contents of V and Nb, metallographic examination disclosed the formation of very large V and Nb carbo nitrites [V, Nb (C, N)]. A steel with a high Mo was also investigated, but after apparently taking a good polish initially, it was found to be unsuitable.
Thus there exists a need for a stainless steel of lens quality, and, in addition, for such a steel which is more economical than the currently used steel in terms of a lower rejection rate, a lower cost per pound, or both, adequate hardness, good wear resistance, excellent through hardenability, good machinability, and excellent corrosion resistance.